Surgeons practicing in the field of neurosurgery have been aware for a number of years of the importance of perforating a patient's cranium at an angle of substantially 90.degree. to a plane tangent to the surface thereof. The purpose behind this requirement is actually twofold: first, a burr hole drilled at an orientation of 90.degree. to the surface of the skull prevents injury to the underlying dura and brain tissue which may otherwise be caused due to the continued rotation of the metal bit of the drilling apparatus once the bit tip exits the skull.
That is to say that, at an orientation of other than 90.degree., the drill bit of drilling devices in current use often continues to rotate after the tip portion has pierced the cranium, despite the incorporation of a standard, safety clutch mechanism commonly utilized with these drilling devices. The operation of this standard clutch is such that the drill should automatically stop rotating once the pressure upon the tip of the bit is released, i.e., once the tip of the bit passes entirely through the cranium.
In current state of the art drills, a secondary drill hole is used to prevent forward motion of a drill bit ("plunging") once the cranium has been penetrated. This secondary hole is made by using two concentric dill bits, that is, a primary bit is positioned inside a secondary bit. The secondary hole forms a platform which prevents forward motion once both the primary and secondary drill bits stop rotating.
The type of drill bit commonly utilized during neurological procedures, however, comprises an outer sleeve and a slideable inner sleeve, partially rotatable within the outer sleeve, which extends a minimal distance beyond the outer sleeve. If the burr hole is oriented at some angle other than 90.degree., while the trailing edges upon the outer sleeve of the bit are still engaging the bony surface of the patient's skull, the grinding surfaces upon the inner sleeve, which extend beyond those of the outer sleeve as noted above, can and often do cause serious injury to the underlying tissue within the cranial cavity.
In contrast, however, a burr hole aligned at an angle of substantially 90.degree. to the surface of the skull permits the clutch mechanism to operate properly and thus stops the rotation of the entire bit once the tip has penetrated the bone, preventing accidental injury to the brain and related tissues.
The second and no less important general purpose behind providing a burr hole at an angle of substantially 90.degree. to the surface of the skull is to ensure that a ventricular catheter subsequently inserted into the brain, perpendicular to the curvature of the cranium, will not deviate from its intended path due to a misaligned skull hole. Standard procedures for ventricular catherization are disclosed in the textbook literature (see, for example, Neurosurgery, edited by Robert H. Wilkins and Setti S. Rengachary, Section A, Chapter 13, "Techniques of Ventricular Puncture" (McGraw-Hill 1984).
A recently developed apparatus and procedure for ensuring correct catheter placement was, in fact, disclosed and claimed by one of the present applicants in U.S. Pat. No. 4,613,324, issued Sep. 23, 1986. The apparatus comprises a guide assembly which, when positioned over an orifice (previously drilled by other means) above the anterior horn of the lateral ventricle in the cranium, guides a catheter and obdurator through the orifice and into the lateral ventricle at an angle normal to an imaginary plane formed by a tangent to the cranium at the orifice.
Although, as discussed above, the importance of orienting one or more burr holes at an angle of substantially 90.degree. to the surface of the patient's skull during such a surgical procedure is well-known, there is currently no relevant art of which the applicants are aware which teaches how this burr hole through the patient's cranium can be prepared and aligned so as to extend through the skull at an angle of substantially 90.degree. to the surface thereof. The difficulty in producing such a precisely aligned burr hole has thus led to the search for a rapid, simple, inexpensive and accurate method and apparatus for perforating the patient's cranium at the required angle.
Another drawback of current perforators is their lack of re-start capability, that is, the ability to re-start the drill if drilling should stop prior to penetration of the cranium.
Current perforators incorporate a straight slot on the drive stem which engages a pin holding the inner and outer sleeves of the perforator together. The inner sleeve is also spring loaded thus allowing it to move along the axis of the perforator. The mechanism works provided that the inner drill is pressed against the drive stem by the pressure of the skull bone against its tip and subsequent compression of the spring. Once this pressure is relieved either by penetration of the inner table of the skull or by the surgeon relaxing the force on the perforator, the spring forces the drive stem to disengage from the inner drill, thereby preventing any further perforator penetration. Thus, the current design requires a surgeon to remove the apparatus from the burr hole and manually re-align the inner and outer drill sleeves with the drive stem before re-drilling can begin. While it is imperative that the perforator stop immediately after penetration of the inner table of the skull to prevent damaging the dura or the cortex, it is also important that the surgeon be able to stop and start drilling before completing the burr-hole.